Numerous high-temperature processes exist in which mass is added to or removed from a substrate. Particular applications include vapor deposition processes such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), which are routinely used in semiconductor wafer processing. Another application is thermogravimetric analysis (TGA) wherein a known mass of a composition is placed in a TGA chamber and heated in accordance with a graduated temperature profile and the change in mass is recorded as a function of change in temperature and/or time.
While atomic-level monitoring of changes in mass can be very valuable in both of these processes, known methods for monitoring the rate of deposition or evaporation of the composition generally lack the capability of providing precise measurements at elevated temperatures. Typically, measurements of changes in mass during a high-temperature process, such as TGA, are performed by a microbalance. Known microbalances typically measure a change in mass by measuring mechanical displacement. In one mass-balancing approach, a pivot is used to balance the weight of the support structure including deposited material with that of reference weights. Alternatively, the weight of the structure can be balanced with a variable electromagnetic force. In yet another embodiment, the compression of a spring upon which the support structure is mounted can be monitored to measure a change in weight,
The precision of these methods is limited because the weight of the structure will generally dwarf that of the material under analysis, thereby making changes in the mass of the material under analysis difficult to detect. Moreover, the existence of convection currents resulting from thermal gradients in the chamber can produce a buoyancy effect that reduces the perceived weight of the support structure.
In a method of this invention, a piezoelectric balance is heated, and the mass of a material deposited thereon is increased or decreased. The change in the mass of a material deposited on the piezoelectric balance is monitored by measuring the frequency response of the piezoelectric balance. Suitable applications include measurements of deposition, evaporation and/or post-growth oxidation or reduction.
An apparatus of this invention includes a deposition chamber or TGA chamber, a piezoelectric balance, and a heater for heating the piezoelectric balance. The piezoelectric balance includes a piezoelectric material that is stable at high temperatures. For example, the piezoelectric material can be a material having a Ca3Ga2Ge4O14-type structure, a member of the (Al, Ga)N system, or gallium phosphate. In preferred embodiments, the piezoelectric material is a composition that can undergo a vibrational resonance when a voltage is applied at temperatures above about 500xc2x0 C. and, more preferably, at temperatures above about 600xc2x0 C. A particularly preferred piezoelectric composition for use in this invention is langasite.
In accordance with one aspect of the invention, a workpiece is positioned in the deposition chamber along with the piezoelectric balance and the two are heated to approximately the same temperature while a film is simultaneously deposited upon each. Deposition can be carried out via chemical vapor deposition (CVD) or physical vapor deposition (PVD). The frequency response of the balance is measured to monitor the rate of deposition on the workpiece. In other embodiments, the frequency response of the piezoelectric balance can be monitored with changing temperature to determine the change in mass of a composition on the balance as a function of temperature. Alternatively, the frequency response of the piezoelectric balance can be monitored at an elevated temperature in a changing atmosphere to determine the change in mass of a composition on the balance as a function of changing gas composition. The frequency response of the balance can be monitored with an impedance analyzer electrically coupled to the balance.
This invention offers several advantages both over the use of conventional piezoelectric balances at or near room temperature and over the use of known methods for measuring change in mass at high temperatures.
First, the use of a high-temperature piezoelectric balance of this invention allows for in situ measurement of changes in mass at high temperatures. In this regard, the materials of this invention provide for operation over a much greater temperature range than that of more-conventional piezoelectrics, such as quartz. Consequently, these materials can be used to accurately measure changes in mass as a function of temperature across these extended ranges or as a function of gas composition at a given elevated temperature. Accordingly, piezoelectric balances of this invention can be used to reliably and accurately measure the rate of deposition in CVD or PVD processes, for example, where the substrate is heated to temperatures over 500xc2x0 C. Similarly, the piezoelectric balances can serve as highly accurate and reliable detectors for monitoring change in mass resulting from oxidation and reduction during high-temperature post-growth annealing.
Second, relative to conventional methods, such as those involving weight measurements, the methods of this invention are capable of measuring changes in mass with remarkably-high precision. With measurements performed in accordance with this invention, frequency changes as fine as one hertz, which translates to a balance resolution of one part per million or finer where the operative frequency is 1-10 MHZ. Further, changes in the thickness of a deposited film can be detected on a monolayer basis, which is particularly useful in applications such as semiconductor processing where deposited layers may be a mere 5 to 10 atoms thick. Accordingly, the piezoelectric balance of this invention can be said to operate as a nanobalance. This level of precision is far superior to that of conventional methods for performing TGA.